You manufacture brain wave entrainment CDs for companies that sell self-improvement products. You are at the post office, with ten boxes of them ready to close up and ship out, but you have a problem. Nine of the boxes contain CDs that are designed to put the listener into an "alpha" or relaxing state, and one is full of Cds that are designed to put the user into a deeper "delta" state, for deep sleep. They look identical, and you forgot to label them.
There is one difference, however. You remember that the "alpha" Cds weigh 13 grams, and because different CD "blanks were used, the "delta" Cds weigh 15 grams. Unfortunately, you can't feel the difference in weight by lifting them.
The post office does have a scale. It costs one dollar each time you weigh something, though, and you want to keep your costs down. How do you use the scale as few times as possible to determine which are the "delta" CDs?
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